This invention relates generally to a steering apparatus for a motor vehicle and, more particularly, to a steering apparatus having steering assist and auxiliary steering features.
In a known steering apparatus of this type (German Unexamined Published Patent Application No. 2,052,085), the steering gear and assisting servomechanism act in the customary manner upon the wheels of the front axle when the auxiliary steering system is switched off, whereas the auxiliary steering system for the wheels of the rear axle is locked hydraulically in the position for traveling straight ahead. In order to obtain all-wheel steering, the auxiliary steering system is switched on by means of a manually operated hydraulic valve in such a manner that the pressure chambers of its steering mechanism used for turning the wheels in the left direction are connected to the appropriate valve connection of the steering valve for a left turn, whereas the pressure chambers of the servomechanism of the steering gear used for a left turn are disconnected from the corresponding valve connection of the steering valve and connected to the pressure chambers of the steering mechanism of the auxiliary steering system which are used for turning in the right direction. Upon this conversion to all-wheel steering, there is no change in the connection of the pressure chambers of the servomechanism of the steering gear used for a right turn to the corresponding valve connection of the steering valve associated with a right turn. As a result of this circuit arrangement, unfavorable conditions are encountered during service with all-wheel steering during left rotation of the hand steering wheel, because the pressure pump which develops the working pressure is then connected exclusively to the steering mechanism of the auxiliary steering system, the pressure chambers of which are used for a right turn are now connected, via the pipe connection established by the manually operated valve, to the pressure chambers of the servomechanism of the steering gear for a left turn. Therefore, the full working pressure for the overall steering assistance for both vehicle axles appears in the steering mechanism of the auxiliary steering system, which is equally as undesirable as the abrupt switching of the pressure pump to the servomechanism of the steering gear when the hand steering wheel is rotated in the right direction. Lastly, due to the purely hydrostatic steering movement of the vehicle axle inherently cooperating with the steering gear from the steering mechanism of the other vehicle axle, the fixed association of the steering angles of the wheels on the vehicle axle with the steering gear to the turn angle of the hand steering wheel also ceases to exist.
The object of this invention is to provide a steering apparatus of the type discussed in which the above-enumerated disadvantages of known steering systems are eliminated.
This and other objects are attained in a steering apparatus which comprises a steering gear, with hydraulic servo assist, for steering the wheels of a first vehicle axle, and a selectable auxiliary steering arrangement which allows the wheels of a second axle to be selectively steered or locked in place for straight ahead travel. A hydraulic pressure transmitting device, which is mechanically connected to a gear output of the steering gear and hydraulically connected to the auxiliary steering arrangement, provides for transmitting hydraulic pressure to the auxiliary steering arrangement when auxiliary steering is selected. The hydraulic pressure transmitted corresponds with movements of the gear output of the steering gear and is independent of the hydraulic pressure supplied to the hydraulic servo device. In a preferred embodiment of the invention, the steering gear includes a first gear assembly which has a gear output connected directly to the wheels of a vehicle axle, and a second steering gear assembly which has a gear output connected to the hydraulic pressure transmitting device. In another preferred embodiment, the gear output of the second steering gear assembly may also be connected directly to the vehicle wheels. The hydraulic pressure supplied to the servo mechanism is preferably limited as a function of the hydraulic pressure transmitted to the auxiliary steering mechanism. This pressure is limited to a predetermined pressure value which may also be adjusted as a function of the hydraulic pressure transmitted to the auxiliary steering mechanism.
In a steering apparatus according to this invention, it is first of all advantageous that the force transmitting device of the hydraulic linkage can be connected at any desired point of the steering linkage connected to the gear output of the steering gear, for example, a steering-gear arm. It is also advantageous that the full pressure for the steering assistance to a plurality of vehicle axles does not appear in the steering mechanism of the auxiliary steering system. Any abrupt change of the connection of the pressure pump to a servo-mechanism of one vehicle axle or to a steering mechanism of another vehicle axle upon a change of the direction of rotation of the hand steering wheel is also avoided in the steering apparatus of the present invention, in which the force transmitting device ensures a fixed correspondence of the steering angles at the relevant vehicle axle with the turning angle of the hand steering wheel in both the main and the auxiliary steering systems.